Recently, demands have been increasing for high density of recording and reproduction with an optical disc or an opto-magnetic disc requiring semiconductor lasers producing short wavelength light. Semiconductor lasers employing a II-VI compound semiconductor have been given attention as producing short wavelength light.
FIG. 4 is a schematic cross-sectional view illustrating a semiconductor laser described in Applied Physics Letters, Volume 59, 1991, at page 1272. The semiconductor laser of FIG. 4 includes an n type GaAs substrate 19, an n type GaAs buffer layer 18 disposed on the n type GaAs substrate 19, an n.sup.+ type ZnSe layer 17 having a thickness of 0.1 .mu.m and a dopant impurity concentration n=1.times.10.sup.18 cm.sup.-3 disposed on the n type GaAs buffer layer 18, and an n type ZnSSe cladding layer 16 having a thickness of 2.5 .mu.m and a Cl (chlorine) dopant impurity concentration of n=1.times.10.sup.18 cm.sup.3 disposed on the n.sup.+ type ZnSe layer 17. An n type ZnSe light confinement layer 15 having a thickness of 1 .mu.m and a carrier concentration n=1.times.10.sup.18 cm.sup.3 produced by Cl doping is disposed on the n type ZnSSe cladding layer 16, a CdZnSe quantum well layer 14 having a thickness of 100 Angstroms is disposed on the n type ZnSe light confinement layer 15, a p type ZnSe light confinement layer 13 having a thickness of 0.1 .mu.m and a carrier concentration p=2.times.10.sup.17 cm.sup.-3 produced by N (nitrogen) doping is disposed on the CdZnSe quantum well layer 14, a p type ZnSSe cladding layer 12 having a thickness of 1.5 .mu.m and a carrier concentration p=1.times.10.sup.18 cm.sup.3 produced by N doping is disposed on the p type ZnSe light confinement layer 13, and a p.sup.+ type ZnSe contact layer 11 having a thickness of 0.1 .mu.m and a carrier concentration p=1.times.10.sup.18 cm.sup.3 produced by N doping is disposed on the p type ZnSSe cladding layer 12. An insulating layer 10 is disposed on the p.sup.+ type ZnSe contact layer 11, a p side electrode 8 comprising Au or Pt or the like is disposed on the insulating layer 11, and an n side electrode 9 is disposed on the n type GaAs substrate 1 and comprises Au-Ge.
In the prior art semiconductor laser shown in FIG. 4, after growing the n type GaAs buffer layer 18 on the n type GaAs substrate 19, the n.sup.+ type ZnSe layer 17, the n type ZnSSe cladding layer 16, the n type ZnSe light confinement layer 15, the CdZnSe quantum well layer 14 serving as an active layer, the p type ZnSe light confinement layer 13, the p type ZnSSe cladding layer 12, and the p.sup.+ type ZnSe contact layer 11 are successively grown by molecular beam epitaxy.
Next, the insulating layer 10, such as polyimide, is formed on the p.sup.+ type ZnSe contact layer 11 and etched, using photolithography, to form a stripe shape opening, the p side electrode 8 is formed in this opening, and the n side electrode 9 is formed on the n type GaAs substrate 19.
In the semiconductor laser shown in FIG. 4, when a forward direction bias is applied across the electrodes 8 and 9, between the n type GaAs substrate 19 and the p.sup.+ type ZnSe contact layer 11, a current flowing through respective layers of the laser is confined by the insulating layer 10 and injected into the CdZnSe quantum well layer 14 serving as an active layer. The injected carriers are confined in the CdZnSe quantum well layer 14 to produce light emission by recombination.
Since the Fermi level, .epsilon.f of FIG. 5, of p type ZnSe is quite far from the valence band, it forms a Schottky barrier about 1 eV or more in height with any metal. Accordingly, in the p/n type semiconductor laser shown in FIG. 4, a Schottky barrier is present between the p side electrode 8 comprising Au, Pt, or the like and the p.sup.+ type ZnSe contact layer 11, as shown in FIG. 5, whereby a preferable ohmic characteristic is not obtained and holes are not injected effectively, increasing the operating voltage.
In a light emitting element having a p on n type structure comprising a p type wide band gap energy II-VI compound semiconductor layer on an n type III-V semiconductor layer, a preferable ohmic contact to the p type II-VI compound semiconductor layer shown in FIG. 4 is not obtained. On the other hand, it is possible to obtain a reasonable ohmic contact to the n type II-VI compound semiconductor layer, so a p type III-V compound semiconductor layer might be disposed on the n type II-VI compound semiconductor layer. However, the growth temperatures of respective layers are 600.degree. to 700.degree. C. for III-V layers and 250.degree. to 400.degree. C. for II-VI layers. Accordingly, when a III-V layer is deposited on the II-VI layer at 600.degree. to 700.degree. C., the II-VI layer is decomposed due to the high temperature or voids occur, making it impossible to deposit a p type III-V layer on the n type II-VI layer.
As a structure for solving this problem, an n/p type structure including an n type II-VI compound semiconductor layer on a p type III-V semiconductor layer as shown in FIG. 6 is described in Applied Physics Letters, Volume 59, 1991, at page 3619. In the laser structure of FIG. 6, a p type GaAs buffer layer 2 having a carrier concentration p=1.times.10.sup.18 cm.sup.3 is disposed on a p type GaAs substrate (not shown). The structure includes a p type ZnSSe layer 22 having a thickness of 1.5 .mu.m and a carrier concentration of p=4.times.10.sup.17 cm.sup.3 produced by N doping, a p type ZnSe layer 23 having a thickness of 0.5 .mu.m and a carrier concentration of 4.times.10.sup.17 cm.sup.3 produced by N doping, and a CdZnSe-ZnSe multi-quantum well layer 21 serving as an active layer. The laser structure also includes an n type ZnSe layer 24 having a thickness of 0.5 .mu.m and a carrier concentration of n=5.times.10.sup.18 cm.sup.3 produced by C1 doping, an n type ZnSSe layer 25 having a thickness of 1 .mu.m and a carrier concentration n=5.times.10.sup.17 cm.sup.3 produced by Cl doping, and an n.sup.+ type ZnSe contact layer 26 having a thickness of 100 nm and a carrier concentration of n=1.times.10.sup.18 cm.sup.3 produced by Cl doping.
In this structure, because an n.sup.+ type ZnSe layer 26 is employed for the contact layer, a reasonably good ohmic contact characteristic is obtained. Here, Ti, Au-Zn, or the like is employed for the p side electrode at the GaAs substrate and In is employed for the n side electrode at the ZnSe layer 26.
The semiconductor laser shown in FIG. 6 may be fabricated as follows. After forming the p type GaAs buffer layer 20 on a p type GaAs substrate (not shown), the p type ZnSSe cladding layer 22, the p type ZnSe light confinement layer 23, the CdZnSe-ZnSe multi-quantum well layer 21 (the active layer), the n type ZnSe light confinement layer 24, the n type ZnSSe cladding layer 25, and the n.sup.+ type ZnSe contact layer 26 are successively grown by MBE.
Next, an insulating film (not shown), such as polyimide, is formed on the n.sup.+ type ZnSe contact layer 26, the insulating film is etched, using photolithography, to form a stripe opening, and an n side electrode comprising In is formed in this opening. On the opposite side of the GaAs substrate, the p side electrode comprising Ti, Au-Zn, or the like is formed.
In the semiconductor laser shown in FIG. 6, when a forward direction bias is applied across the electrodes between the p type GaAs substrate and the n.sup.+ type ZnSe contact layer 26, injected carriers are confined in the CdZnSe-ZnSe multi-quantum well layer 21 and produce light emission by recombination.
In the prior art semiconductor laser shown in FIG. 6, however, an energy band discontinuity is caused by the band gap energy difference and electron affinity difference between the p type GaAs layer 20 and the p type ZnSSe cladding layer 22, whereby, as shown in FIG. 7, a spike S about 1.4 eV in height is formed in the valence band, thereby preventing the injection of holes into the active layer 21. Accordingly, in both n/p type and p/n type structures, the operating voltages are similarly high.